skype: provide more info about file transfers

Till now only a single line was printed, when a new offer arrived. Now
two more lines are printed:

- when the actual transfer starts (offer accepted), with destination
  filename
- when the transfer completes
make check: fix this to work again
test skype_parse_filetransfer()
Implement 'group info' command to list members of a group (#741)
test skype_parse_group()
skype: move config instructions from README to the manpage
Merge from vmiklos. Adds a "group info" command to list members of a group.
skype: mention --plugindir quirk in HACKING

As skype support always gets built as plugin, option is required
for running tests without installing plugin to the system.
skype.test: do proper cleanup of skyped process despite possible exceptions, keep pexpect.log
skype: make chdirs work more reliably between tests
skype: use argparse instead of getopt for CLI in skyped
skype: add --dont-start-skype option

For cases when case skype start/stop/restart is being handled externally.
Skype4Py seem to be able to handle client going on/off.
skype: add ATTACH_GDB env flag, extend hacking instructions with info on pexpect/gdb
skype: add newline to all the lines emitted
skype: cleanup of the send() code

Use socket.sendall(), as send() is not guaranteed to send all
the data passed (though it should generally work that way with
blocking sockets).
Use more robust, obvious and idiomatic loop conditions.
Cleanup redundant imports and variables.
skype: don't do retries on send() on socket errors with gobject

As it's a blocking socket, I imagine there might be only EINTR
errors from signals, which don't seem to be used here.
I think the same tweak can be applied to select() code, but
I don't run it, so no idea if Skype4Py threads might make such loop
necessary there.
skype: handle socket errors during tls session negotiation in a graceful way

Before this patch, such errors (which happen 1/4 times here) lock
skyped forever, producing traceback (and hanging because threads
in python).
Proper fix would be to see why these happen (might be ssl handling
in the plugin), but that's no excuse not to handle socket errors
without crashing the daemon.
skype: handle only socket errors in send()
skype: check for truncated commands on receiving